Ratings agency Moody's downgraded 15 of the world's biggest banks last Thursday, lowering credit ratings by one to three notches to reflect the risk of losses they face from volatile capital markets activities

Testifying before a Senate committee on Wednesday, JPMorgan Chase CEO Jamie Dimon said he was “dead wrong” when he dismissed early reports of the bank’s $2 billion trading loss as “a tempest in a teapot.”